Recently, as the dietary habits are being westernized and also living habits are being changed in Japan, patients suffering from diseases caused by the living habits such as diabetes, hyperlipemia, hypertension and obesity are increasing in number. For the purpose of preventing people from these diseases, the demand of various healthful foods such as Foods for Specified Health Use is increasing and, in addition, the physiological function or action of foods has lately attracted attention. Dietary fibers known to have physiological actions such as controlling action of intestinal function and also controlling action of postprandial hyperglycemia are generally used as materials for improving the function of foods.
The dietary fibers can be divided into two groups, i. e. water-soluble dietary fibers and water-insoluble ones. These two groups of dietary fibers are different from each other in the physical properties and physiological properties. Because the water-insoluble dietary fibers such as cellulose and hemicellulose are difficultly assimilated in the large intestine and these fibers containing water are excreted in feces, the fibers exhibit physiological actions of increasing the quantity of the feces and shortening the passing time in the gastrointestinal tracts. The water-soluble dietary fibers can be classified into a group of those which exhibit a high viscosity when they are dissolved in water, such as pectin, psyllium and guar gum and a group of those which are not viscous when they are dissolved in water, such as hardly digestible dextrin and polydextrose. The water-soluble dietary fibers having a high viscosity form a gel in the digestive tracts to retard the absorption of nutrients by the inhibition of the diffusion. For example, when the absorption of saccharides is retarded, an increase in the blood sugar level can be controlled and, accordingly, excess secretion of insulin can be effectively prevented. When the blood sugar level and insulin secretion after meals can be controlled, it is expected to improve the glucose tolerance and also the lipid metabolism in the long run. Further, when the lipid absorption is retarded, the elevation of neutral fat level after meals is controlled to exert an influence on the lipid metabolism. In addition, the discharge of bile acid is promoted by the shortening in the passing time in the digestive tracts and increase in quantity of feces. The effects thus obtained are that the sterol group in the body is reduced and that cholesterol level in the serum is lowered. However, it is difficult to add an effective amount of highly viscous dietary fibers to foods because such an additive exerts an influence on the taste, texture, shape, etc. of the foods. Thus, the foods in which the dietary fibers can be added are limited.
On the other hand, as the water-soluble dietary fibers of a low viscosity, hardly digestible dextrin (dietary fiber-containing dextrin) produced from starch is known. Patent Document 1 discloses a method for producing hardly digestible dextrin by reacting pyrodextrin with α-amylase. Patent Document 2 discloses a method for producing dextrin having a high dietary fiber content by reacting pyrodextrin with α-amylase and then with glucoamylase and then collecting the dietary fibers by the chromatographic fractionation and also a method for increasing the dietary fiber content by the reaction with transglucosidase prior to the chromatographic fractionation. Because the physical properties of the hardly digestible dextrin are suitable for the additives for foods, this kind of dextrin is widely used as dietary fiber material for foods. The physiological actions of the hardly digestible dextrin include, for example, controlling actions of intestinal function (refer to Patent Document 3), preventing action from obesity or glucose tolerance troubles by the addition thereof to foods such as sugar (Patent Document 4), controlling action of insulin secretion (Patent Document 5), lowering action of serum lipid component content (Patent Document 6) and lowering action of high blood pressure (Patent Document 7). The hardly digestible dextrin is used for healthful foods such as Foods for Specified Health Use.
However, the hardly digestible dextrin is colored by the roasting at a high temperature in the course of the steps of producing it. Although it can be decolored to some extent in a purification step, the complete decoloration is difficult and the final commercial product thereof is in the form of pale yellow powder. Thus, when the hardly digestible dextrin is added to foods such as those the coloring of which causes a problem, e.g. water, transparent soft drinks and rice, the foods are colored to lower the commercial value of them. For this reason, the amount of the hardly digestible dextrin is limited. It was thus difficult to use the hardly digestible dextrin in such an amount that the physiological function thereof can be expected. Although the hardly digestible dextrin can be added to foods having a deep color such as tea drinks, soups and miso soup without any problem immediately after the production, the foods are gradually colored brown with time to damage the stability of the appearance of these commercial products. Another problem is that when the hardly digestible dextrin is used in combination with other sweetening agents, the foods having a neutral pH are easily browned in the course of the production thereof and, in addition, the foods are easily scorched in the course of the boiling down.
On the other hand, Patent Document 8 discloses hardly digestible starch syrup or powdered starch syrup obtained by hydrolyzing pyrodextrin in the presence of an acid has a physiological action and that such a syrup can be boiled down and is usable for foods in a wide range. However, the hardly digestible starch syrup or powdered starch syrup has a high saccharide content and, accordingly, it has a high degree of sweetness. Thus, such syrup cannot be used for saccharide-free foods or foods which must have no sweetness, while it can be added to foods which contain saccharides.
For solving the problem of the coloring of hardly digestible dextrin and increase in the degree of coloring thereof with time, it is known to hydrogenate the reducing end thereof because the coloring is caused by Maillard reaction (browning). When the reducing end is sealed, the hardly digestible dextrin cannot react with amino acids and, therefor, Maillard reaction does not occur. This technique is well known and, in fact, hydrogenated dextrin obtained by hydrogenating dextrin (refer to Patent Document 9), hydrogenated, hardly digestible starch syrup obtained by hydrogenating the hardly digestible starch syrup (refer to Patent Document 10), etc. are known. After the hydrogenation, dextrin thus hydrogenated has a lowered calorie and the digestion and absorption thereof are slowed down. However, when the hydrogenated dextrin is taken in together with other saccharides, it does not exert any influence on the digestion and absorption of these saccharides. Other physiological functions of the hydrogenated dextrin have not yet been confirmed. As for the hardly digestible starch syrup, only its non-cariogenic function was confirmed but other physiological functions thereof have not yet been elucidated. It is also known that hydrogenated, hardly digestible dextrin obtained by hydrogenating the hardly digestible dextrin is free from the change in color with time and that this dextrin has a refreshing taste. However, the health benefit of this hydrogenated, hardly digestible dextrin has not yet been elucidated (Patent Documents 1 and 2).
Under the above-described circumstances, it is expected to develop and also to commercialize a hardly digestible substance free from the above-described defects of the hardly digestible dextrin, capable of being added to any kind of foods without the problem of the coloring, and having physiological functions equal to or better than various physiological functions of the hardly digestible dextrin.    [Patent Document 1] Japanese Patent Kokai No. Hei 2-145169    [Patent Document 2] Japanese Patent Kokai No. Hei 2-154664    [Patent Document 3] Japanese Patent No. 2007645    [Patent Document 4] Japanese Patent Kokai No. Hei 6-166622    [Patent Document 5] Japanese Patent No. 2007644    [Patent Document 6] Japanese Patent No. 2007646    [Patent Document 7] Japanese Patent No. 2019839    [Patent Document 8] Japanese Patent Kokai No. Hei 11-116602    [Patent Document 9] Japanese Patent Kokai No. Hei 5-214002    [Patent Document 10] Japanese Patent Kokai No. Hei 10-150934